Abstract

Biomagnetic monitoring, using tree leaves as passive surfaces for particle collection, has been shown to be a promising technique for assessing the dispersion and deposition of particles in the context of anthropogenic
pollution. By comparing leaves' magnetic properties with trace metal levels measured in the leaves, we here assess the utility of the biomagnetic technique as a sensitive, fast and inexpensive method for assessment of volcanic plume deposition. Samples of sweet chestnut leaves (Castanea sativa) were collected from the area surrounding Mt. Etna volcano in Sicily during the 2008 growing season when the volcano was displaying mild eruptive activity. Previous work has shown that the trace metal concentrations of these leaves show promise as a bio-indicator of volcanic gas, aerosol and ash deposition on the flanks of Mt. Etna. For 2008, ICPMS analysis of the elemental abundances within the leaves showed that As, Cd, Cu, Mo, Tl, K, B, Al and Co displayed elevated concentrations downwind of the volcanic source, to the E–ESE, but with overall reduced concentrations relative to 2007. Less explosive activity than 2007 and a broader, more easterly wind field may have distributed the volcanic plume over a wider area, both of which would account for reduced trace element concentrations in 2008. Correspondence of elevated concentrations in both years (2007 and 2008) with their respective wind fields suggests that plume deposition is the controlling factor rather than variability in the soils and that these leaves do indeed have potential as bio-indicators of the plume's
dispersion. Magnetic analysis of the leaves shows that the spatial distribution of saturation isothermal remanent magnetisation (SIRM) and magnetic susceptibility (χlf) values display a strong correlation with the wind-influenced plume transport direction for 2008, with elevated concentrations of magnetic minerals on the eastern flanks, in broad agreement with the ICP-MS data. This spatial distribution provides further evidence of plume variability as the main control on the concentrations of magnetic particles on the leaf surfaces and on elemental uptake by the tree and suggests that biomagnetic monitoring may also hold promise as a method of assessing the dispersion and impacts of volcanic plumes. The dominant magnetic mineral on the leaf surfaces is a magnetite-like mineral (contributing >90% of the SIRM), of coarse, multidomain
(MD) grain size (∼5 to 15μm). A volcanogenic source is most likely, as magnetites from anthropogenic sources are typically an order of magnitude smaller in grain size (∼0.1–1 μm). These new results from the use of environmental magnetism on leaves, in a volcanically-affected region, suggest that biomagnetic monitoring offers a new, sensitive and rapid means for the assessment of volcanic plume deposition over wide areas.